What causes smoking Caravans?

Dear Car Talk

Dear Car Talk | Feb 01, 1992

Dear Tom and Ray:

Our 1987 Dodge Caravan (40,000 miles, 2.6 liter, automatic) exudes malodorous blue and black smoke and idles roughly when first started. After two to three minutes, it runs normally. Our mechanic says this is oil leaking past the valve guides and is characteristic for the engine. He can fix it, but says it won't hurt anything and that any repair won't last. His advice is to simply be sure to watch the oil level and continue regular oil changes. Can you suggest a more permanent repair, other than a new car? Is the "valve guide" explanation consistent with your experience?

RAY: In our experience, Tom, valve guides aren't usually the cause of smoking Caravans. In fact, black AND blue smoke usually indicates that you have two problems.

TOM: When you see black smoke, that means the engine is running too rich (i.e. too much gas, not enough air). And when you see blue smoke, that's a sign that oil is burning.

RAY: If you had seen white smoke, that would have meant a new Pope had been elected, but we've never seen that on anything but Italian cars.

TOM: Seriously, Tom, here's what I think is happening. Your choke is set too rich. So when you start the car in the morning, too much fuel pours into the cylinders. That explains the black smoke and the rough idling for the first few minutes.

RAY: And believe it or not, the blue smoke may also be caused by the choke problem. Here's what happens. When too much gas pours into the cylinders, not all of it burns. Much of it sneaks down past the rings and washes away the motor oil on the cylinder walls. That's bad. Reeeeeall bad.

TOM: Besides the possibility of permanent damage to the pistons and cylinder walls, what that unburned gas is doing is continually diluting the engine oil. That thinner, diluted oil, can then work its way up past the rings back into the combustion chamber, where it's burned (and makes that pretty plume of blue smoke you see out your rear view mirror).

RAY: So getting the choke fixed will definitely stop the black smoke, and may do quite a bit to stem the blue smoke, too. If the blue smoke persists--even after a couple of oil changes--that means the diluted oil prob?ably did allow damage to be done to the rings and cylinders. In that case, you're going to continue to burn some oil.

TOM: So get the choke fixed, change the oil, and hope for the best, Tom. And keep an eye on your house in the rear view mirror as you drive away in the morning. As long as you can still see it through the blue smoke, you're probably OK.

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